The present invention relates to a scintillation camera for obtaining a picture of distribution of a radioactive isotope (RI).
Conventional scintillation cameras, which were developed by H. 0. Anger and are in common use today, have the disadvantage that they cause the images to be distorted and that they are not sensitive enough.
One of two representative types of the Anger scintillation camera is equipped with a lattice type collimator as shown in FIGS. 1 and 2, while the other is equipped with a parallel plate collimator which is rotatable in construction as shown in FIGS. 3 to 5.
In the case of the scintillation camera shown in FIGS. 1 and 2, radioactive rays are collimated by the lattice type collimator 1 so that only those which are incoming perpendicularly to the surface of a flat type scintillator 2 (i.e., which are incoming in the direction of the Z axis) may be fed to the scintillator 2. A plurality of photomultipliers 4 arranged on the back of the scintillator 2 emit outputs which correspond to the scintillations of light applied to the photomultipliers through optical couplings or light guides 3. These outputs are fed to a position calculation circuit 5, in which the positions of scintillations (i.e., the positions on which the gamma rays are incident) are calculated from these outputs so that position signals X and Y may be developed. In a display unit 6, dots are displayed in the positions determined by the position signals X and Y. These dots gather in the picture and indicate a distribution of RI as it is viewed in the direction of the Z axis. Thus the specific organ 8 of a subject 7, in which RI is allowed to collect, can be diagnosed.
The trouble is that radioactive rays are emitted from the RI in all directions, and therefore it is not too much to say that gamma rays are not being efficiently utilized if they are admitted into a scintillation camera only from a single direction. In order that a scintillation camera may be highly sensitive, not only the gamma rays which are incoming in the direction of the Z axis but also those which are incoming in the oblique direction, have to be detected by the scintillator 2, not to speak of those which are incoming in the direction of the Z axis but are obstructed by the thickness of the strips of which the lattice type collimator 1 is made. This requirement is met by a rotating-collimator type scintillation camera.
The rotating-collimator type scintillation camera can be constructed from an ordinary scintillation camera. For this purpose, the lattice type collimator 1 (FIG. 1) is replaced by a parallel plate collimator 11 (FIG. 3) which is rotatably held, adapted to be rotated by a suitable mechanism, and connected to a computer 12 through interface circuits 13 and 14 as shown in FIG. 4. The angle of piecemeal rotation is specified by the computer 12. Every time the collimator 11 has rotated by the specified angle, signals X and Y are taken into the computer 12 and stored in the memory circuit thereof. On the other hand, the signals X and Y developed by the position calculation circuit 5 of the scintillation camera are listed as the picture elements of a two-dimensional matrix (FIG. 5). Since the coordinate axes of this matrix cannot be resolved, coordinate transformation is required for obtaining data on one-dimensional distribution from the abovedescribed data on two-dimensional distribution. Let it be supposed that that the X axis of the collimator 11 points to the direction of the arrow x as shown in FIG. 5. Then the angular difference between the X axis of the collimator 11 and that of the matrix is calculated by the computer 12 so that coordinate transformation may be effected by making necessary addition to the discrete value for each picture element defined by the X axis of the matrix. Thus a large number of data on one-dimensional distribution are obtained, which are subjected to image information processing so as to be reconstructed into a two-dimensional image. In order to improve the accuracy of a reconstructed image, the two-dimensional matrix must be sufficiently fine-meshed and the collimator 11 must be adapted to be rotated by as small angular distances as possible so that data on one-dimensional distribution may be obtained in as great numbers as possible. Let it be supposed that, in order to meet this requirement, data on one-dimensional distribution are going to be obtained from a two-dimensional matrix having a mesh size of 512 .times.512 every time the collimator 11 rotates by 0.5 degree in the course of 180-degree rotation. Then, two-dimensional data on 360 pictures in the original coordinate system must be stored, which means that the number of memory locations required will amount to as large as 512.times.512.times.360 and that the operation for the abovementioned coordinate transformation will place a large burden on the computer 12.
Under these circumstances, the first object of the present invention is to provide a scintillation camera which is based on a different principle from that of the conventional scintillation cameras and into which a computerized technique of image information processing is introduced so as to free the images from distortion and allow the camera to be highly sensitive and to have a high practical spatial resolution.
The second object of the present invention is to improve the Anger scintillation camera so as to make the high accuracy of reconstructed images compatible with a smaller number of data to be stored and thereby simplify the computer operation to such an extent that the time interval from the instant the operation is started until the instant an image is delivered may be reduced.
The third object of the present invention is to provide an anisotropic collimator for use in a scintillation camera, ECT, or the like, this collimator being of such simplified construction that a collimator having any degree of effectiveness as a field stop can be easily manufactured.